1. Field of the Invention
This invention relates to an adhesive for printed circuit board, a method of producing a printed circuit board by using the same and a printed circuit board, and more particularly to an adhesive having improved resistance to chemicals, heat resistance, electrical properties and adhesion property, a method of easily producing a printed circuit board by using such an adhesive and a printed circuit board having a high reliability.
2. Disclosure of the Related Art
Recently, high densification or high speed access of calculating function in electronic equipment such as large-size computer or the like is advanced with the progress of electronic techniques. As a result, high densification and high reliability through fine pattern are lately required even in printed circuit board and LSI mounted printed circuit board. Particularly, multilayer printed circuit boards having a plurality of printed circuit layers are lately spotlighted in accordance with high densification and high-speed access.
In the production of the printed circuit board, there has hitherto been known a so-called etched foil method in which a copper foil is laminated onto a substrate and subjected to a photoetching to form a conductor circuit as a method for the formation of conductor circuit. According to this method, the conductor circuit having an excellent adhesion property to the substrate can be formed, but there is a serious drawback that it is difficult to obtain a fine pattern of a high accuracy by etching because the thickness of the copper foil is thick. Furthermore, the production steps become complicated and also the production efficiency is poor.
As a method of forming the conductor circuit onto the substrate, therefore, there has recently been adopted a so-called additive method wherein an adhesive including a diene series synthetic rubber is applied to the surface of the substrate to form an adhesive layer and then the surface of the adhesive layer is toughened and subjected to an electroless plating to form a conductor circuit.
However, the adhesive usually used in the latter method includes the synthetic rubber, so that the heat resistance is low because the adhesion strength is largely lowered at, for example, high temperature and the electroless plated film swells during the soldering, and the electric properties such as surface resistivity and the like are insufficient, and hence the use range is fairly restricted.
As a typical example of the multilayer printed circuit board, there is a system that plural circuit plates each provided with an internal circuit pattern are laminated with each other through a prepreg as an insulating layer and pressed and then a through-hole is formed therein to connect and electrically conduct the internal circuit patterns to each other.
In the multilayer printed circuit board of this system, the plural internal circuit patterns are connected and electrically conducted to each other through the through-hole, and the wiring circuit pattern becomes too complicated, so that it is difficult to realize the high densification or high speed access.
For this end, there are lately developed multilayer printed circuit boards, in which conductor circuit and organic insulating films are alternately built up on each other, as a multilayer printed circuit board capable of overcoming the above problem. Such a multilayer printed circuit board is certainly suitable for extra-high densification and high speed access.
However, it is actually difficult to form an electroless plated film on the organic insulating layer with a good reliability. In the multilayer printed circuit board, therefore, the conductor circuit is formed by PVD process such as vapor deposition, sputtering and the like, or by using electroless plating with the above PVD process. However, the formation of conductor circuit through PVD process is poor in the productivity and undesirably high in the cost.
On the other hand, the inventors have made various studies in order to solve the aforementioned problems produced in the above adhesives for electroless plating, and previously proposed an adhesive for electroless plating having improved heat resistance, electrical properties and adhesion property to the electroless plated film and capable of relatively easily utilizing it and a method of producing circuit boards by using the adhesive and printed circuit boards produced therefrom (Japanese Patent laid open No. 61-276875, No. 63-126297, No. 2-182731, No. 2-188992, and U.S. Pat. No. 4,752,499, No. 5,021,472 and No. 5,055,321).
That is, these techniques relate to an adhesive in which heat-resistant resin powder being soluble in an oxidizing agent and subjected to a curing treatment is dispersed into an uncured heat-resistant resin matrix being insoluble in the oxidizing agent through the curing treatment, and a method of producing printed circuit boards in which the above adhesive is applied to a substrate and dried and cured to form an adhesive layer, and thereafter at least a part of the above powder dispersed in the surface portion of the adhesive layer is dissolved and removed to roughen the surface of the adhesive layer, and then the thus treated substrate is subjected to an electroless plating as well as printed circuit boards produced by this method.
In the adhesive according to these techniques, the fine powder of the previously cured epoxy resin is dispersed into the heat-resistant resin matrix, so that when the adhesive is applied to the substrate and then dried and cured, the heat-resistant resin fine powder is at a uniformly dispersed state in the heat resistant resin forming the matrix. Since there is a difference in the solubility in the oxidizing agent between the heat-resistant resin fine powder and the heat-resistant resin matrix, the fine powder dispersed in the surface of the adhesive layer is mainly dissolved and removed by treating the adhesive layer with the oxidizing agent to effectively form an anchor recess, whereby the surface of the adhesive layer can be toughened uniformly and hence the high adhesion strength and high reliability to the electroless plated film can be obtained.
In the above adhesive, easily available epoxy resins having excellent resistance to chemicals, heat resistance, electrical properties and hardness are used as the cured heat-resistant resin fine powder soluble in acid or oxidizing agent.
However, it has been confirmed that when the printed circuit board produced by using epoxy resin as a heat-resistant resin fine powder is used in an atmosphere of high temperature and humidity, dissolution and precipitation of copper forming the wiring circuit pattern are caused to lower the value of surface resistivity and hence a short circuit is formed between the patterns with the rapid advance of high densification in the printed circuit board.
An ionic compound is used in the production of epoxy resin powder soluble in acid or oxidizing agent, so that sodium ion or chlorine ion remains in the epoxy resin fine powder and causes migration reaction as shown in the following reaction formulae (1)-(3) (see FIG. 15): EQU Na.sup.+ Cl.sup.- +H.sub.2 O.fwdarw.NaOH+HCl (1) EQU Cu+2NaOH.fwdarw.Cu(OH).sub.2 +2Na.sup.+ ( 2) EQU Cu+2HCl.fwdarw.CuCl.sub.2 +2H.sup.+ ( 3)
Thus, in the conventional printed circuit boards, there is still a problem that the reliability is lowered in accordance with the use condition due to the use of the epoxy resin inevitably containing sodium ion and chlorine ion.